The introduction of multigigabit, multiwavelength optical communication systems operating over long distances (e.g., transoceanic) and high average powers has resulted in the exploration of fiber designs that can minimize signal degradation. Fibers in such systems typically have losses in the range of about 0.20 to 0.25 dB/km. To increase bandwidth, fibers need to be redesigned to reduce a number of nonlinear and polarization effects that become increasingly important at high bit rates and high powers. In particular transmission performance is limited by a number of phenomena, including accumulation of amplified spontaneous emission (ASE) noise, dispersion, and a nonlinear component to the refractive index of the fiber. For amplitude modulated digital signals temporal distortions due either to dispersion or Kerr effect are a significant contributing factor to the bit error rate (BER).
A design solution to minimize the effects of dispersion and Kerr effect nonlinearities is not simple. The obvious solution to dispersive waveform distortions is to have the fiber dispersion set to zero at the signal wavelength. When signals travel at the zero dispersion wavelength they do not suffer any temporal distortions. However, a signal traveling at the zero dispersion wavelength and ASE noise generated by the optical amplifiers travel at similar velocities so that there is good phase matching, and thus they have the opportunity to interact over long distances, via the Kerr effect. The result is the transfer of power out of the signal and into unwanted wavelengths. Similarly, if two, or more signal channels are located around the zero dispersion wavelength they will be well phase matched and thus interact strongly. As a result of the interaction, energy will be transferred from one signal to another leading to waveform distortions. Conversely if the signal propagates at a wavelength for which the dispersion is large then there is a large phase mismatch (i.e., a group velocity difference) between the signal and noise or two adjacent signal channels, which greatly reduces the efficiency of four wave mixing. However, large values of dispersion result in increased inter-symbol interference due to the temporal spreading of the signal.
In summary, the following competing system design factors need to be taken into account. If the fiber has non-zero chromatic dispersion, then pulse spreading results, with attendant intersymbol interference. On the other hand, if chromatic dispersion is zero everywhere along the transmission fiber then non-linear effects such as four-wave-mixing will cause power transfers between signal channels as well as between signal and noise. In either case the result is a degradation in system performance.
An important advance in the implementation of multi-channel WDM systems has been the use of dispersion management techniques. In view of the above mentioned conflicting demands, the basic principle of dispersion management is to keep local dispersion non-zero but make the overall system dispersion substantially zero. This can be accomplished by using a dispersion map in which the zero dispersion wavelengths of the constituent fibers are chosen so that they are appropriately far from the system's operating wavelengths. Constituent fibers with different zero dispersion wavelengths are then arranged in some periodic fashion so that the path average dispersion for the whole transmission line is appropriately small. For example, the transmission line may be divided into two or more sections approximately equal in length. In one section, the optical fiber has a zero dispersion wavelength less than the operating wavelengths. The following section has optical fiber with a zero dispersion wavelength greater than the operating wavelengths. The overall transmission line is thus constructed in a periodic manner from a concatenation of fiber sections having different zero dispersion wavelengths. By constructing the transmission line out of alternating lengths of positive and negative dispersion fiber, the path average dispersion can be adjusted so that it causes minimal temporal distortion. Moreover, by selecting the local dispersions of the constituent fibers to be large in magnitude, and making the period of the dispersion map an appropriate length, nonlinear interactions can be suppressed. The path-average dispersion of a fiber span of length L may be mathematically denoted as:Daverage=∫D(z′)dz 
For applications involving the transmission of non-return-to-zero (NRZ) data, the desired Daverage is zero, while, for soliton data transmission, the desired Daverage is in the range of about 0.05 to 0.5 picoseconds per nanometer-kilometer.
The principle of using large local values of dispersion to suppress nonlinear interactions and concatenating segments of such fiber with opposite delay values to ensure temporal fidelity of the signal is sound. However the transmission line is a periodic structure; the regularly spaced amplifiers give rise to periodic fluctuations in the signal energy. This introduces an asymmetry to the FWM and XPM between signal channels as they traverse the line that can actually enhance these processes. So despite the use of dispersion maps to suppress nonlinear interactions, FWM and XPM can still be problematic. The periodicity of the transmission line can resonantly enhance FWM between channels that were otherwise poorly phase matched. The periodicity can, under certain circumstances, also enhance XPM between two otherwise benign channels.